Method for protection of evaporator heating elements from corrosion

ABSTRACT

The corrosion of heating elements in evaporators for alkaline media is substantially prevented by using chromium steel containing preferably 25-28 percent chromium as the metal for the heating elements in the presence of 0.05 to 1g/1 of sodium chlorate.

United States Patent Orlova et al.

[451 May23,'l972 METHOD FOR PROTECTION OF EVAPORATOR HEATING ELEMENTSFROM CORROSION Frida Abramovna Orlova, Leninsky prospekt, 41, kv. 334,Moscow; Georgy Vasilievich Seleznev, ulitsa Mayakovskogo, 10, kv. 10;Petr Veniaminovich Bonjuk, prospekt 40 let Oktyabrya, 2, kv. 5, both ofDzerzhinsk, all of USSR.

Filed: May 7, 1970 Appl. No.: 35,564

Inventors:

US. Cl ..l48/6.14, 21/2.7, 23/184,

Sakiyama et al., Chemical Abstracts Vol. 682240700 2/5/68 PrimaryExaminer-Ralph S, Kendall AttameyWaters, Roditi, Schwartz & Nissen [57]ABSTRACT The corrosion of heating elements in evaporators for alkalinemedia is substantially prevented by using chromium steel containingpreferably 25-28 percent chromium as the metal for the heating elementsin the presence of 0.05 to 1 g/ 1 of sodium chlorate.

4 Claims, No Drawings METHOD FOR PROTECTION OF EVAPORATOR HEATINGELEMENTS FROM CORROSION This invention relates to methods for corrosionprotection of heating elements in evaporators operating in alkalinemedia, for example, in evaporators used for the manufacture of causticsoda. The aforesaid evaporators are intended for carrying out theprocesses of concentrating (evaporating) alkaline solutions underelevated temperatures and diverse service conditions, such as natural orforced circulation of alkaline solutions, various pressures of heatcarrier (heating steam), ing a significant proportion of solid matter,etc.

During the service life of evaporating plant equipment the principalelements of evaporators, viz., heating tubes, undergo maximumdeterioration under the effect of corrosive alkaline media and alsosuffer from pronounced erosion corrosion, particularly where evaporatoroperation involves forced circulation of alkaline solutions containingdifferent admixtures.

Diversified methods for corrosion protection of heating elements ofevaporators intended for handling alkaline media are known in the art.

For example, use is made of heating elements manufactured fromcorrosion-resisting materials, e.g.nickel, nickel-base alloys, orhigh-alloy austenitic steels which contain, apart from chromium,molybdenum and other alloying elements, also an adequately highproportion of nickel, viz. 25-28 percent.

In some instances recourse is had to equipment made from ordinary steel,provided diverse inhibitors, such as sodium nitrate or saccharose, areincorporated into the alkaline corrosive medium.

It is further known to effect anode polarization technique.

The known methods for corrosion protection suffer from the drawback ofresorting to heating elements made from nickel, which is a criticalmetal, or from nickel-base alloy and nickel-bearing steels, theemployment of nickel for this purpose being particularly undesirable inview of the constantly growing consumption of nickel for the productionof heat-resistant and high-temperature alloys. This situation hasprompted world-wide studies directed to the development of steels andalloys containing a lower percentage of nickel and to findingconditions, under which steels containing little or no nickel wouldexhibit corrosion-resisting properties for specific applications.

The aforesaid studies are essentially aimed at minimizing theconsumption of nickel for the equipment to be used in large-tonnagemanufacturing processes, among which mention may be made of caustic sodaproduction, under temperature conditions which do not necessarilyheat-resistant materials.

The known methods involving the use of corrosion inhibitors are notinvariably effective, particularly where the equipment to be protectedis made from carbon steel.

The employment of anodic protection in heating elements is likewiseinefficient due to the principles on which the design of these elementsis based.

It is an object of the aforesaid disadvantages.

It is a further and more specific object of the present invention toprovide a method for corrosion protection of heating elements ofevaporators handling alkaline media which will made it possible toimprove substantially the dependability and service life of heatingelements and to make the heating elements from non-critical materials.

According to the invention, the objects have been accomplished by theprovision of heating elements made from chromium steels which undergopassivation under the effect of an inhibitor, sodium chlorate, presentin a concentration of 005-1 g/l in an alkaline medium at a temperatureof 90-450 C.

A protective film formed as a result of the interaction between chromiumsteel and a chlorate-containing alkaline solution exhibits highadhesiveness to the base metal and is corrosion protection by thepresent invention to eliminate the erosion due to the circulation ofalkaline pulp containalso noted for its good resistance to erosiveeffects caused by a rapid stream of hot alkaline medium (stream rate,4-5 m/sec containing about 20 percent of solid phase, e.g. sodiumchloride in the manufacture of caustic soda.

No incorporation of the corrosion inhibitor (sodium chlorate) inalkaline solution is required when manufacturing caustic soda, since inthis case sodium chlorate is the byproduct of caustic soda manufactureand is present in the alkaline solution in an amount of 0.05-1 g/l whichis adequate for chromium steel passivation.

Where no sodium chlorate is present in alkaline solution, theaforespecified amount of. the inhibitor should be incorporated into thesolution being handled.

Currently available equipment manufactured from chromium-nickel steelcontaining 18% Cr and 10% Ni can be used in conjunction with the heatingelements, according to the present invention, since the contact betweenthe aforesaid steel and chromium steel (25% Cr) is permissible and aheating element may be assembled by flaring chromium steel tubes in atube sheet made from chromium-nickel steel.

It is also preferable to use chromium steel or carbon steel clad withchromium steel for fabricating tube sheets of heating elements.

Data listed in Table l pertains to laboratory experiments under staticconditions in a 50 percent solution of sodium hydroxide and made itpossible to evaluate the behavior of various steel grades and of nickelin an alkaline environment at temperatures that are close to those usedfor the evaporation of alkaline solution under plant conditions.

it follows from the above data that among the steels tested chromiumsteel is unique in that it exhibits high corrosion resistance incorrosive alkaline medium throughout the range of test temperatures.

Under the same conditions, carbon and chromium-nickel steels undergodissolution at a high rate and the presence of an oxidizing component(sodium chlorate) exerts a marked accelerating effect on the rate ofcorrosion.

Hence, carbon steel and also chromium -nickel steel andchromium-nickel-molybdenum steel at elevated temperatures Table 2 Steelgrade Starting Sodium Temp. Specimen specimen chlorate C weight weight,content loss dug. g/l ring test period, 180 hrs, in grams.

Steel containing It is apparent from the data of Table 2 that even athigh temperatures the inhibiting action of sodium chlorate is retained,the rate of the specimen corrosion in melted alkali in the presence ofsodium chlorate being 20-23 times lower than without sodium chlorate.

With reference to chromium steels containing a lower percentage ofchromium 13 and 17% Cr), it is evident that these materials retain anadequate corrosion resistance and undergo passivation in a 30 percentsolution of sodium hydroxide containing sodium chlorate.

Table 3 summarizes the results of laboratory tests on chromium steelspecimens containing 13 and 17% Cr. Corrosion resistance was evaluatedat different temperatures in a 30 percent solution of sodium hydroxidecontaining sodium chlorate.

Table 3 Steel Content Corrosion rate, mm/year grade of sodiumTemperature, C

chlorate,

Steel 0.0 0.6 1.3-1.8 containing 13% Cr 0.35-0.45 0.19 0.77 Steel 0.00.24 0.45 0.60 containing 17% Cr 0.35-0.45 0.07 0.08 0.15-0.20 0.68

It is apparent from the data of Table 3 that increasing the degree ofalloying the steel with chromium results in broadening the rangeoftemperatures at which this type ofsteel retains its ability to undergopassivation in a 30 percent solution of sodium hydroxide.

For a better understanding of the present invention, presentedhereinbelow are the following examples of plant tests on specimens ofdiverse metals placed in the tubes of heating elements in evaporators atcaustic soda plants. Tests were also conducted on tubes made fromappropriate metals and flared in the tube sheets of heating elements inevaporators used for evaporating electrolytic caustic soda fromdiaphragm cells.

EXAMPLE 1 Plant tests were conducted on specimens made from diversesteel grades, viz., chromium-nickel steel containing 18% Cr and Ni;chromium-nickel-molybdenum steel containing 18% Cr, 12% Ni and 3% Mo;and chromium steel containing 17% and 25% Cr, in a medium consisting of42 percent solution of sodium hydroxide, in which the content of sodiumchlorate equals 0.8-1 g/l and the content of sodium chloride equals 20g/l, the temperature of the heating steam being 120 C.

Apart from the corrosive action exerted by the medium, the testspecimens undergo erosion caused by a high-speed stream of alkaline pulpV stream 4-5 m/sec) Under the aforespecified conditions, the steelspecimens are corroded at the following rate, millimeters per year: 7

Steel containing 18% Cr and 10% Ni 2.5 Steel containing 18% Cr,

l2% Ni, and 3% M0 1.8 Steel containing 25% Cr 0.06 Steel containing 17%Cr 1.2

EXAMPLE 2 carbon steel tubes 2 to 3 chromium-nickel and chromium-nickelmolybdenum steel tubes 6 to 8 1n the tubes made from chromium steel,continuous service resulted in neither corrosion damage of the tubes nordiminution of the tube wall thickness. At the sites of flaring the tubesin the tube sheets of heating elements there developed no leaks.

The present method is useful not only for corrosion protection ofevaporator heating elements, but also for corrosion protection ofequipment handling alkaline media under the aforesaid conditions indiverse chemical processes.

What we claim is:

1. In a method of dehydrating an aqueous alkaline solution byevaporating water from said solution at a temperature of to 450 C. whilein contact with metal surfaces, the improvement which comprises adding0.05 to lg/l of sodium chlorate to the alkaline solution and utilizing ametal consisting of chromium steel containing 25 to 28 percent ofchromium.

2. In a method of dehydrating an aqueous alkaline solution containingchlorate ions by evaporating water from said solution at a temperatureof 90 to 450 C. while in contact with metal surfaces, the improvementwhich comprises utilizing a metal consisting of chromium steelcontaining 25 to 28 percent chromium.

3. A method according to claim 1 wherein the aqueous alkaline solutionis an aqueous solution of sodium hydroxide.

4. A method according to claim 2 wherein the aqueous alkaline solutionis an aqueous solution of sodium hydroxide and sodium chloride.

2. In a method of dehydrating an aqueous alkaline solution containingchlorate ions by evaporating water from said solution at a temperatureof 90* to 450* C. while in contact with metal surfaces, the improvementwhich comprises utilizing a metal consisting of chromium steelcontaining 25 to 28 percent chromium.
 3. A method according to claim 1wherein the aqueous alkaline solution is an aqueous solution of sodiumhydroxide.
 4. A method according to claim 2 wherein the aqueous alkalinesolution is an aqueous solution of sodium hydroxide and sodium chloride.